Yeah, that’s the thing with gasses like nitrogen, helium or carbon monoxide: it feels like breathing air except since it contains no oxygen you’ll just go from feeling light headed to passing out and suffocating. CO2 would otherwise work the same way except that inhaling pure CO2 instantly causes panic and the sense of suffocation.
Infact, I’ve heard of people who euthanize pet rodents with CO2 and recommend others doing the same which is terrifyingly bad advice. That’s a fucking awful way to go. Just imagine being so ignorant and doing that to your pet. Makes me feel sick to even think about it.
Carbon monoxide similarly replaces the oxygen in your red blood cells. There are even suicide tutorials online by using carbon monoxide as it’s a painless way to go.
Also, this is not advice and it’s also potenttially lethal to the people coming to save you. Don’t do it and if you do, for the very least, leave a sign warning others about it.